fs: dedup handling of struct filename init and refcounts bumps

No functional changes.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20250313142744.1323281-1-mjguzik@gmail.com


Signed-off-by: Christian Brauner <brauner@kernel.org>